Olefins, particularly ethylene and propylene, are important chemical feedstocks. Typically they are found in nature or are produced as primary products or byproducts in mixtures that contain saturated hydrocarbons and other components. Before the raw olefins can be used, they usually must be separated from these mixtures.
Currently, separation of olefin/paraffin mixtures is usually carried out by distillation. However, the similar volatilities of the components make this process costly and complicated, requiring expensive distillation columns and energy-intensive processing. Jarvelin reports that the fractional distillation of propylene/propane mixtures is the most energy-intensive distillation practiced in the United States (Harri Järvelin and James R. Fair, Adsorptive separation of propylene/propane mixtures, Ind. Eng. Chem. Research 32 (1993) 2201-2207). More energy conserving separation processes are needed.
Membranes have been considered for the separation of olefins from paraffins as an alternative to distillation. However, the separation is difficult largely because of the similar molecular sizes of the components. Another difficulty is that the feed stream conditions are typically close to the gas/liquid phase boundary of the mixture. Also, the membrane must operate in a hydrocarbon environment under conditions of high pressure and temperature. Such harsh conditions tend to adversely affect the durability and stability of separation performance of many membrane materials. For example, some contaminants plasticize selectively permeable membrane materials and can cause loss of selectivity and/or permeation rate. A membrane with sufficiently high olefin/paraffin selectivity, and sufficient durability in long-term contact with hydrocarbon streams under high pressure and temperature is highly desired.
Membrane materials for separating olefinic hydrocarbons from a mixture of olefinic and saturated hydrocarbons have been reported, but none can be easily or economically fabricated into membranes that offer the unique combination of high selectivity and durability under industrial process conditions.
For example, several inorganic and polymer/inorganic membrane materials with good propylene/propane selectivity have been studied. See M. Teramoto, H. Matsuyama, T. Yamashiro, Y. Katayama, Separation of ethylene from ethane by supported liquid membranes containing silver nitrate as carrier, J. Chem Eng. Japan 19 (1986) 1, and R. D. Hughes, J. A. Mahoney, E. F. Steigelmann, Olefin separation by facilitated transport, in: N. N. Li, J. M. Calo (eds.), Membrane Handbook, Van Nostrand, New York, 1992. Such materials are difficult to fabricate into practical industrial membranes. Liquid facilitated-transport membranes have been demonstrated to have attractive separation performance in the lab, but have been difficult to scale up, and have exhibited declining performance in environments typical of an industrial propylene/propane stream.
Solid polymer-electrolyte facilitated-transport membranes appear more amenable to fabrication into stable thin film membranes. See Ingo Pinnau and L. G. Toy, Solid polymer electrolyte composite membranes for olefin/paraffin separation, J. Membrane Science, 184 (2001) 39-48. Such a membrane is exemplified in U.S. Pat. No. 5,670,051 (Pinnau et al, 1997) wherein a silver tetrafluoroborate/poly(ethylene oxide) membrane exhibited ethylene/ethane selectivity of greater than 1000. However, these membranes are severely limited by their poor chemical stability in the olefin/paraffin industrial environment.
Carbon hollow-fiber membranes have shown promise in laboratory tests (“Propylene/Propane Separation”, Product Information from Carbon Membranes, Ltd., Israel), but are vulnerable to degradation caused by condensable organics present in industrial streams. Moreover, carbon membranes are brittle and difficult to form into membrane modules of commercial relevance.
Membranes based on rubbery polymers typically have olefin/paraffin selectivity too low for an economically useful separation. For example, Tanaka et al. report that the single-gas propylene/propane selectivity is only 1.7 for a polybutadiene membrane at 50° C. (K. Tanaka, A. Taguchi, Jianquiang Hao, H. Kita, K. Okamoto, J. Membrane Science 121 (1996) 197-207) and Ito reports a propylene/propane selectivity only slightly over 1.0 in silicone rubber at 40° C. (Akira Ito and Sun-Tak Hwang, J. Applied Polymer Science, 38 (1989) 483-490).
Membranes based on glassy polymers have the potential for providing usefully high olefin/paraffin selectivity because of the preferential diffusivity of the olefin, which has smaller molecular size than the paraffin.
Glassy polymers already used in gas separation have generally shown only modest olefin/paraffin selectivity. For example, Ito has reported that films of polysulfone, ethyl cellulose, cellulose acetate and cellulose triacetate exhibit propylene/propane selectivity of 5 or less (Akira Ito and Sun-Tak Hwang, Permeation of propane and propylene through cellulosic polymer membranes, J. Applied Polymer Science, 38 (1989) 483-490).
U.S. Pat. No. 4,623,704 describes a process utilizing a cellulose triacetate membrane for recovering ethylene from the reactor vent of a polyethylene plant. However, the vent stream that contained 96.5% ethylene is moderately upgraded to only 97.9% in the permeate stream for recycle to the reactor.
Membrane films of poly(2,6-dimethyl-1,4-phenylene oxide) exhibited pure gas propylene/propane selectivity of 9.1 (Ito and Hwang, Ibid.) Higher selectivity has been reported by Ilinitch et al. (J. Membrane Science 98 (1995) 287-290, J. Membrane Science 82 (1993) 149-155, and J. Membrane Science 66 (1992) 1-8), but the values at higher pressure were uncertain and were accompanied by undesirable plasticization of the membrane by propylene.
Polyimide membranes have been studied extensively for the separation of gases and to some degree for the separation of olefins from paraffins. Lee et al. (Kwang-Rae Lee and Sun-Tak Hwang, Separation of propylene and propane by polyimide hollow-fiber membrane module, J. Membrane Science 73 (1992) 37-45) disclose a hollow fiber membrane of a polyimide that exhibited mixed-gas propylene/propane selectivity in the range of 5-8 with low feed pressure (2-4 barg). The composition of the polyimide was not disclosed.
Krol et al. (J. J. Krol, M. Boerrigter, G. H. Koops, Polyimide hollow fiber gas separation membranes: preparation and the suppression of plasticization in propane/propylene environments, J. Membrane Science. 184 (2001) 275-286) report a hollow fiber membrane of a polyimide composed of biphenyltetracarboxylic dianhydride and diaminophenylindane which exhibited a pure-gas propylene/propane selectivity of 12; however, the membrane was undesirably plasticized by propylene at propylene pressure as low as 1 barg.
Polyimides based on 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and aromatic diamines have been found to provide a favorable combination of propylene permeability and propylene/propane selectivity. Permeation data for dense-film membranes of two different 6FDA-containing polyimides have been reported to have pure gas selectivity for propylene/propane in the range of 6-27. (C. Staudt-Bickel et al, Olefin/paraffin gas separations with 6FDA-based polyimide membranes, J. Membrane Science 170 (2000) 205-214). Higher selectivity for similar 6FDA polyimides has been reported in U.S. Pat. No. 5,749,943 (Shimazu et al); however, it is anticipated that mixed-gas selectivity at high pressure will be much lower due to plasticization by the propylene-rich feed gas.
U.S. Pat. Nos. 4,532,041; 4,571,444; 4,606,903; 4,836,927; 5,133,867; 6,180,008; and 6,187,987 disclose membranes based on a polyimide copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful for liquid separations.
U.S. Pat. Nos. 5,605,627; 5,683,584; and 5,762,798 disclose asymmetric, microporous membranes based on a polyimide copolymer derived from the co-condensation of benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and a mixture of toluene diamines useful for liquid filtration or dialysis membranes.
U.S. Pat. No. 5,635,067 discloses a fluid separation membrane based on blends of phenylindane-containing polyimide polymers with polyimides derived from the condensation of benzophenone-3,3′,4,4,′-tetracarboxylic acid dianhydride (BTDA) with toluenediisocyanate (TDI) and 4,4′-methylene bisphenylisocyanate (MDI) and/or polyimides derived from the condensation of BTDA and pyromellitic dianhydride with TDI and MDI.
A significant shortcoming of published data for the separation of olefins from paraffins using membranes is the absence of data under practical industrial conditions: e.g., high feed and permeate pressure and high temperature. These are conditions under which plasticization of the membrane material could become significant and which could result in substantial decline in membrane performance over extended periods of time. In spite of the considerable efforts to provide industrially viable membranes for the separation of olefins from paraffins, none has proven to meet the performance criteria required for industrial application.